A patent document 1 discloses a vibration suppressing technique to calculate a vibration suppressing torque for suppressing sprung vibrations by using, as input variables, a driving torque and a wheel speed, and to decrease a control gain when a vibration amplitude of the vibration suppressing torque continues to be greater than or equal to a predetermined amplitude for a predetermined time length.
However, patent document 1 provides no explanation on restoration or return after the decrease of the control gain. Therefore, there might be a possibility of reoccurrence of hunting after restoration to the control. Moreover, a stoppage of the operation of the vibration suppressing control for a longer period of time to prevent reoccurrence of hunting may cause a decrease of frequency of performing the vibration suppressing control.